Within the last fifty years, efforts have been expended by the Navy, Army and the Air Force to develop a system for detection and discrimination of targets among clutter (be it on the surface of the sea or in land) as well as a means for stabilizing the aimpoint of tracking radars. A typical system comprises a transmitter and a two-channel receiver where the transmitter emits radiation of a selected polarization and the receiver receives two orthogonal polarization simultaneously on a polarization-insensitive antenna.
A fully polarimetric radar system requires a transmitter that is capable of radiating both polarization senses (horizontal and vertical or, equivalently, right circular and left circular) and a receiver that is capable of receiving and processing both polarization senses of the reflected scattered energy simultaneously. A typical such system may transmit alternate radiations of vertical and horizontal polarizations, for example, by means of waveguide ferrite switches. On receive, orthomode transducers derive separate horizontal and vertical polarization senses from a common polarization-insensitive antenna. The orthogonal signals are then processed in a two-channel receiver. Such fully polarimetric radars generate an enormous amount of data which is desirable but the necessary signal processing in real-time requires ultra-complex processors.
Many applications of polarimetric signal processing in weapon systems can tolerate only limited hardware and signal processing complexity. Therefore, as a compromise, polarization-sensitive architectures that do not yield the full complement of scattering matrix are often used. These systems generally transmit only one sense of polarization with the two-channel receiver receiving and processing the co-polarized and cross-polarized back-scattered energy simultaneously. Consequently, the two channels must be phase and amplitude-tracked through the final detector, resulting in a complex and expensive radar receiver front-end for the missile seeker on which such a radar system may be mounted.
The volume and cost of smart munitions often further exclude the wide use of multi-channel receivers. However, the target data contained in both the cross and co-polarized return energy is still desirable for accurate target identification among the clutter. One solution could be using receivers which separate the orthogonal polarizations, then time-multiplexing these signals into a common receiver channel. But the orthomode transducers which are required in such a solution are generally built from hybrid waveguide structures or by the use of reflective antennas with diagonal grid polarizers--both bulky components.